1. Field of the Invention
The present invention relates to a toner for developing an electrostatic latent image formed by electrophotography, electrostatic recording methods, electrostatic printing methods, etc. In addition, the present invention relates to a method for manufacturing the toner. Further, the present invention relates to an image forming method and apparatus using the toner.
2. Discussion of the Background
As for electrophotography, various methods have been disclosed, for example, in U.S. Pat. No. 2,297,691 and Japanese Patent Publications Nos. (hereinafter referred to as JPPs) 49-23910 and 43-24748. In these methods, a copy image is typically formed by the processes of forming an electrostatic latent image on a photoreceptor including a photosensitive material by one of various latent image forming methods; developing the latent image with a toner; transferring the toner image on a receiving material such as papers; and fixing the toner image thereon upon application of heat, pressure or a solvent vapor.
The methods for developing an electrostatic latent image are broadly classified into liquid developing methods using a liquid developer in which one or more of pigments or dyes are finely dispersed in an insulating organic liquid, and dry developing methods, such as cascade developing methods, magnetic brush developing methods and powder cloud developing methods, which use a dry developer including a toner in which a colorant such as carbon black is dispersed in a natural or synthetic resin. Currently, the dry developing methods are mainly used because dry developers are easier to handle than liquid developers.
As for fixing methods in electrophotography, heat roller fixing methods have been typically used because of having good energy efficiency. However, these heat roller fixing methods have a drawback in that a so-called offset phenomenon tends to occur such that when a toner image is fixed, part of the toner image adheres to a heat roller and the part of the toner image is re-transferred on a copy paper, resulting in formation of an undesired image.
In attempting to solve such an offset phenomenon, methods in which a release agent such as waxes is included in a toner have been disclosed. For, example, Japanese Laid-Open Patent Publications Nos. (hereinafter JOPs) 51-143333, 57-148752, 58-97056 and 60-247250 have disclosed to use solid silicone varnishes, higher fatty acids, higher alcohols, various waxes, etc., as a release agent.
However, when such a release agent is included in a toner, a filming problem tends to occur in that the release agent is separated from the toner when developing processes are performed and then the thus formed free release agent adheres to a photoreceptor and a developing sleeve. The thus formed film of the free release agent gradually grows when copying processes are repeated, resulting in formation of a white stripe image on a half tone image when the film becomes too thick. Therefore, it has been needed for a toner including a release agent to solve the offset phenomenon and filming problem at the same time. In other words, in order to produce images having good image qualities for a long period of time, it has been needed to improve toners while paying attention to the release agent therein.
Currently, a need for high quality images increases more and more in the market. Satisfactory images cannot be produced by conventional toners, which typically have a volume average particle diameter of from 10 to 15 μm, and therefore a need exists for a toner having a smaller particle diameter. However, when the particle diameter of a toner becomes smaller, the release agent included in the toner is easily separated from the toner if the toner is subjected to stresses, etc. In particular, when the toner is prepared by a pulverization method, kneaded toner constituents tend to be divided at the position in which the release agent is present when pulverized, which typically has a sharp molecular weight distribution peak and is brittle, and thereby the release agent tends to be present on the surface of the resultant toner particles or tends to be present as fine powders in the resultant toner. Therefore, when a toner having a small particle diameter is used to produce high quality images, the filming problem tends to occur. In addition, another problem of poor fixing occurs. Namely, when a toner having a small particle diameter is used, the amount of the toner included in a half tone image formed on a receiving material decreases. In this case, the amount of heat applied from a heating member to the toner particles transferred on a recessed portion of the receiving material is very small, and therefore the toner image has poorly fixed particularly when fixed at a low temperature.
In addition, in order to save power consumption and reduce CO2 emission, there are needs for image forming apparatus to reduce the waiting time (warm-up time), i.e., an interval between the time when the image forming apparatus is in an on state to the time when an image forming operation can be performed, and to minimize the energy consumed in the pre-heating state of the apparatus in which the fixing unit is preliminarily heated so as to be quickly heated to a temperature in the fixable temperature range when receiving a print order.
There is a technique procurement project for next-generation copiers in the DSM (Demand-side Management) program of the year 1999 of International Energy Agency, and requirements for the copiers are disclosed. As for the requirements for a copier having a copying speed not less than 30 cpm, the waiting time is not greater than 10 seconds and the power consumption in the waiting period (i.e., a period in which image forming operations are not performed) is from 10 to 30 W, which depends on the copying speed. Thus, there are stringent requirements for the next generation copiers to drastically reduce total electric power consumption and CO2 emission.
A method in which a heat roller having a low heat capacity is used to improve its temperature sensitivity is considered to be useful for satisfying the requirements. However, the method is not satisfactory because in medium-speed to high-speed image forming apparatus, the amount of heat applied to toner images is very small.
In order to fulfill the above-mentioned requirements, i.e., to minimize the waiting time, it is considered that to lower the fixable temperature of the toner itself is needed.
When the level of low temperature fixing is discussed referring to the preset fixing temperature of the image forming apparatus, the image forming apparatus are needed to have a preset fixing temperature lower than the preset fixing temperature at which a current low fixable toner can be used. Therefore it is considered that a toner satisfying the requirements cannot be easily developed only by using known techniques.
In addition, when a toner having low temperature fixability is used, it is considered that to impart a wide fixable temperature range (i.e., a hot-offset resistance) and a good high temperature preservability to the toner become difficult.
In particular, when the image forming speed increases, the temperature of the heating member tends to decrease in fixing, resulting in decrease of the amount of heat used for fixing. Therefore, measures should be taken on both the toner and fixing device sides to improve low temperature fixability.
In attempting to solve these problems, various methods have been proposed. For example, JOP 5-173354 discloses a toner including a release agent, wherein the physical properties, addition amount and species of the release agent are specified. JOP 6-161144 discloses a toner including a release agent, wherein the shape of the release agent dispersed in the toner is specified. JOP 7-104500 (i.e., Japanese Patent No. 2,675,974) discloses a toner having a specific coefficient of dynamic friction and including an external additive treated with a polyalkylene and a silicone oil to improve the cleaning ability and durability of the toner.
JPP 8-3656 discloses a toner system including two different magnetic toners having different coefficients of dynamic friction. JOP 11-95477 discloses a toner including an external additive having a coefficient of dynamic friction of from 0.12 to 0.30 to improve the transferability of the toner and to avoid the filming problem. JOP 2000-105484 discloses a technique in which the coefficient of static friction of a toner image is specified to improve the fixability of the toner image. JOP 2000-310875 discloses a toner whose coefficient of friction is specified when the external additive of the toner is embedded into the toner by stresses. In addition, JOP 2001-5220 discloses four color toners for forming full color images, wherein the coefficients of dynamic friction are specified.
However, both the filming problem and the offset problem can be solved at the same time by these toners and methods. In particular, when the toners are subjected to mechanical stresses and heat stresses in image forming apparatus, the effects thereof are hardly exerted, i.e., the filming problem occurs when used for long period of time. Therefore, a need exists for a technique by which the filming problem and the offset problem can be avoided at the same time even when a toner is used for a long period.
In attempting to improve low temperature fixability of toner, several methods in which two different polyester resins having different properties are used in a toner have been proposed. For example, a method in which anon-linear polyester resin is used in combination with a linear polyester resin (JOP 60-90344); a method in which a crosslinkable polyester having a glass transition temperature (Tg) not lower than 50° C. and a softening point not higher than 200° C. is used in combination with a linear polyester resin having a softening point not higher than 150° C. and a weight average molecular weight (MW) of from 3,000 to 50,000 (JOP 64-15755); a method in which a non-linear polyester polymer having a weight average molecular weight not less than 5,000 and a variance ratio (MW/MN) not less than 20 is used in combination with a non-linear polyester polymer having a weight average molecular weight of from 1,000 to 5,000 and a variance ratio not less than 4 (JOP 2-82267); a method in which an organic metal compound including a linear polyester                resin having an acid value of from 5 to 60 and a non-linear polyester resin having an acid value less than 5 is included in a toner (JOP 3-229264); a method in which a first saturated polyester resin is used in combination with a second saturated polyester resin having an acid value 1.5 or more times the acid value of the first polyester resin (JOP 3-41470); etc.        
In these methods, it is intended to achieve a good combination of low temperature fixability and hot offset resistance by using a mixture of a non-crosslinkable resin with a crosslinkable resin. However, the blended resins have good compatibility (i.e., the resins can be mixed well with each other), and therefore the kneaded toner constituents cannot be easily pulverized, resulting in deterioration of productivity, and thereby the manufacturing costs increase.
In addition, toners in which a polyester resin having good fixability is mixed with a styrene-acrylic resin having good pulverizability have been disclosed in JOPs 49-6931 and 54-11424. However, since polyester resins typically have poor compatibility with styrene-acrylic resins, both the resins are unevenly dispersed in a toner when the resins are simply mixed mechanically. Therefore when a toner is prepared, a colorant such as carbon black and a charge controlling agent are poorly dispersed in the toner, resulting in occurrence of fouling in the background areas of the resultant toner images.
JOPs 4-142301 and 7-98517 have proposed methods in which a resin prepared by polymerizing a polyester resin with a styrene resin in a container is used for a toner to impart a good pulverizability to the toner and to uniformly disperse a colorant and a charge controlling agent in the resin. However, these toners have a narrower fixable temperature range than in the case in which a non-linear resin and a linear resin are used in combination. In attempting to solve this problem, JOP 8-320593 discloses a toner which includes a mixture of three different resins having different physical properties, i.e., different glass transition temperatures. When preparing this toner, toner constituents cannot be dispersed well because the resins are merely blended, and thereby problems occur such that background fouling is caused in the resultant images and in addition the toner image has poor blocking resistance.
As mentioned above, currently low temperature fixing are increasingly performed, and therefore a need exists for low temperature fixing technique. In addition, a need also exists for a miniaturized image forming apparatus. However, there is no toner which has a low temperature fixability and a wide fixable temperature range (i.e., good hot offset resistance) as well as good high temperature preservability and good pulverizability, and is capable of producing images having good image qualities without producing background fouling and the filming problem even when a low pressure fixing device is used.
Because of these reasons, a need exists for a toner fulfilling the requirements mentioned above. In addition, a need exists for an image forming method and apparatus, in which high quality toner images are produced for a long period of time without causing the filming problem and offset problem while electric power consumption and waiting period are reduced.